1. Field of the Invention
The present invention relates to an optical encoder for use in, e.g., a displacement sensor.
2. Description of the Related Art
A Talbot encoder or a triple grating encoder is known as a detector that can detect a position or an angle using a small and relatively simple configuration.
For example, as disclosed in Specification of U.S. Pat. No. 5,774,219 (FIG. 1 and FIG. 2), the triple grating encoder is constituted of a light source, a transmissive light source scale, a main scale, and a photodiode array for light receiving elements. The main scale is disposed on a measurement target that is relatively displaced with respect to the light source, the light source scale, and the photodiode array. Further, a fixed interval is provided between the pair of the light source scale and the photodiode array and the main scale as a position at which an interference pattern is coupled.
Light emitted from the light source passes through the light source scale, and it is reflected by the main scale. The reflected light couples the interference pattern on a photodiode array surface. The light source, the pair of the light source scale and the photodiode array, and the main scale relatively move. With this movement, the interference pattern coupled with the photodiode array changes. Therefore, a relative position can be detected from a change in interference pattern. Therefore, high-resolution displacement measurement is carried out.
The Talbot encoder or the triple grating encoder has advantages, e.g., a good accuracy as compared with encoders adopting other systems. However, light from the light source must be reflected or transmitted by the scale, and the light must be received at the position where the interference pattern is coupled. Therefore, arrangement of each of the light emission points of the light source or the like or a light projecting portion including an exit region, the scale, and a light receiving portion including a light receiving region of the light receiving element and the like is restricted. That is, the Talbot encoder or the triple grating encoder has a problem that a degree of freedom in arrangement of the light projecting portion, the scale, and the light receiving portion is lowered.
When the degree of freedom in arrangement of the light projecting portion, the scale, and the light receiving portion is low in this manner, intervals between the scale, the light projecting portion, and the light receiving portion cannot be narrowed, for example. Therefore, this matter leads to a problem that the encoder cannot be further thinned.